In the agricultural field, the control of plant growth is an important technology for improving productivity. At present, various kinds of plant growth regulators for the inhibition of plant growth are in practical use and contribute to the improvement of yield of crops and quality of products.
However, the number of plant growth regulators which promote root growth is small, and the regulators have insufficient effects and undesirable effects in many cases. For example, auxin-like substances, which are now widely used as a root promoter, may induce undesirable effects such as leaf epinasty, stem twist, stem breaking, induction of root knots, and plant death depending on the type and condition of a plant or the concentration of the compound to be applied. Therefore, the use method and use amount are limited, and the effect of promoting root growth is insufficient.
Further, in a gramineous plant such as wheat or rice, there is a problem in that, even if flowering and fertilization occurs normally, the subsequent weather condition or the like may make the progress of ripening insufficient, resulting in a decrease in yield. However, the number of plant growth regulators for improving such phenomenon is small, and the effects of the regulators are insufficient.
Meanwhile, phenyllactic acid is known to have a root promoting effect on plants (Non-patent Document 1), but the effect is weak. Therefore phenyllactic acid itself is not in practical use as a plant growth regulator. In addition, phenyllactic acid produced by a lactic acid bacterium (Enterococcus faecalis) has been reported to have antibacterial activity (Patent Document 1).
Moreover, tryptophan has been reported to serve in a specific tissue of a specific plant as a precursor of indoleacetic acid which is a plant hormone (Non-patent Document 2). However, in general, such effect is not provided, and the compound is not in practical use as a plant growth regulator.    Patent Document 1: JP-A-2000-300284    Non-patent Document 1: Mikami et al. 1970. Several synthetic hydroxy-acids as plant growth regulators. Agricultural and Biological Chemistry 34: 977-979.    Non-patent Document 2: Law 1987. Gibberellin-enhanced indole-3-acetic acid biosynthesis: D-Tryptophan as the precursor of indole-3-acetic acid. Physiol. Plant. 70: 626-632.